This application claims priority of German Patent Application No. 197 40 141.4, filed Sep. 12, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for determining a torsional structure in the surface roughness of a finished shaft journal. A method of this generic type is described, for example, in Rau et al., "Drallstrukturen geschliffener Dichtflachen beurteilen", Werkstatt und Betrieb, (November 1997).
As shown in FIG. 10, in order to provide a reliable sealing function at the points where shafts pass through housing walls (1), it is necessary to take into account the properties of both the sealing ring (2) provided with an annular radial sealing lip (3), and those of the opposed running surface (4) on the shaft journal side. As a rule, circumferentially ground journal surfaces are involved. (Other methods of finishing include burnishing, roller-burnishing, external abrasion, and finishing on a lathe.) In addition to specific roughness values, the designer also prescribes that the grinding structure be free of torsion for the shaft journals. In this case, the term "torsion-free" means that the surface structure that results from grinding is aligned precisely in the circumferential direction and there are no superimposed regular wave components.
It has also been assumed heretofore that plunge-cut grinding produces torsion-free structures. However, even the unreliable "thread" method (described below) shows that, at least with a certain combination of operating parameters, the plunge-cut grinding method can cause torsional structures to develop on workpiece surfaces finished in this fashion.
The radial sealing lip of a sealing ring abuts the surface of the shaft journal with a specific radial force imposed by a rubber-elastic sealing edge up to a certain axial width. As a result of the rotation of the shaft journal, the contact area of the sealing lip is deformed to varying degrees in the circumferential direction as a function of the local radial application pressure. Lesser deformations take place near the edge and the greater circumferential deformations occur more in the middle area of the application strip. This results in a sensitive tribological and rheological equilibrium with an oil flow that guarantees the lubrication of the contact zone as well as the sealing function of the annular seal maintained by a recycling mechanism as oil is drawn in and pumped back, thus producing an oil circuit. This equilibrium must not be disturbed by an imposition of torsion in the microstructure of the opposite running surface. A delivery effect for the oil caused by torsion in one direction or the other must be avoided. With a torsion-produced delivery effect in the sealed interior of the housing, the seal would run dry. Contamination from outside would be conveyed into the contact zone and the seal would wear prematurely and leak. An outwardly directed delivery effect would prevent the seal from running dry, but would cause oil to escape at the sealing point, which must be prevented more or less strictly for a variety of reasons.
In the past, these properties were monitored, if at all, by the so-called thread method, which yielded only highly unreliable results, which were frequently not even noticed. In the thread method, an oil-saturated thread of a specific structure, material, and thickness is wrapped around the top of the horizontally aligned shaft journal, over slightly more than 180.degree.; the ends that hang down are fastened jointly to a small weight and the thread is subjected to a specific load in this fashion. The shaft is then turned twenty times slowly in one rotational direction and then twenty times in the other direction. The distance traveled axially by the thread on the surface of the journal is used as a measure of the torsional structure.
The thread method does deliver a clear measurement result. Comparative measurements by the applicant using the thread method and the present invention have shown, however, that the measurement results obtained by the thread method are in no way representative of the actual torsional structure of the journal surface. The measurement results obtained by the thread method also show no correlation whatever with the observable tightness results or the lifetimes of installed radial shaft sealing rings.
The goal of the present invention is to develop a method for determining a torsional structure in the surface roughness of a finished shaft journal that permits qualitatively and quantitatively assured statements regarding the formation and severity of the development of a torsional structure in the surface of a shaft journal.
This goal is achieved according to the present invention by detecting the microstructure of the journal surface at a plurality of measuring points, with a much higher point density being selected in the axial direction than in the circumferential direction. With a suitable choice of the display scales for the individual directions in space (e.g., elevation or elongation in the radial or axial directions and upsetting in the circumferential direction), the topography of a torsional structure becomes clearly visible in a three-dimensional representation of the measurement points.
The advantages of the present invention consist in the fact that for the first time a torsional structure on finished journal surfaces can be rendered clearly and reliably visible. With a greater manifestation of torsion, the most important characteristics such as angle of twist, direction of spiral, pitch, and the like, can be determined from such a primary representation of the topography. If additional parameters must be obtained, for example the delivery cross section, or if the torsion involved is less severely pronounced, a surface autocorrelation function can be formed from the representation of the measured points. The secondary topography thus obtained constitutes an image that is similar to the primary topography in which however stochastic (i.e., random) roughness components are eliminated and essentially only the periodic components are retained, thereby showing the periodicity of the twist (i.e., waviness).
Because every type of factor that causes torsion produces a different appearance of the microstructure of the surface, even the causes of the development of the torsion can be determined by the trained "reader" from the representation of the microstructure of the surface. By virtue of the experience gained with the present invention, it is possible to distinguish for example dressing torsion and offset torsion as the two most important forms of torsion that can develop when cylindrical surfaces are ground.